Tropical Storm Harvey strengthened into a hurricane on Thursday as it headed to coastal Texas, where it was expected to bring life-threatening flooding, meteorologists said.

The National Hurricane Center said that Air Force hurricane hunter planes had spotted Tropical Storm Harvey strengthening in the Gulf of Mexico, with about 60-mile-per-hour winds more than 300 miles off the southern coast of Texas early Thursday. A hurricane warning, meaning hurricane conditions are expected, had been in effect from Port Mansfield in far southern Texas to Matagorda, about 100 miles southwest of Houston.

But later in the day, the planes spotted the system strengthening, and the estimated maximum sustained winds increased to close to 100 m.p.h. the hurricane center said early Friday. It was expected to make landfall Friday night or early Saturday.

Parts of Louisiana and the lower Mississippi Valley were also in the path of hurricane conditions from the storm system, the hurricane center said. Gov. John Bel Edwards of Louisiana said on Thursday that Hurricane Harvey was expected to hit Corpus Christi early Saturday as a Category 3 hurricane, but that its outer bands of rain could be felt earlier. The impact on Louisiana was expected early next week, he said.

“It is getting more and more serious as time goes by,” Mr. Edwards said in a news conference. He said the weather service had told him there was a possibility the hurricane could make landfall in Texas and then re-enter the Gulf before moving east to Louisiana, he said.

Hurricane watches were also in place on Thursday, as were tropical storm warnings, in other parts of the state, mostly along the coast, the National Hurricane Center said.

The movement of the hurricane was expected to slow while over land, meaning it could drop more rain. “This is going to play out over the next week or so,” Mr. Edwards said. “This is a very serious storm.”

In some areas, the storm could unleash at least two feet of rainfall accumulation that could cause “flooding catastrophe” through the weekend, a meteorologist said.

Texas and Louisiana have been bracing for the storm to affect populated areas for days, after the National Hurricane Center predicted this week that it could make landfall, affecting Corpus Christi, Tex., Houston and New Orleans, among other cities.

When the National Oceanic and Atmospheric Administration issues a hurricane warning through its hurricane center, it means that conditions including sustained winds of 74 m.p.h. or higher are expected in the next 36 hours, allowing for preparations by state authorities and residents.

On Thursday, Texas escalated its readiness level for the storm, saying that even as a tropical depression, Harvey posed a threat of imminent disaster, including severe flooding, storm surge and damaging winds, in at least 30 counties.

Gov. Greg Abbott announced that he was pre-emptively declaring a state of disaster for those counties, a formal declaration that allows Texas to quickly deploy resources for any emergency. He encouraged Texans to get ready.

“As the State Operations Center increases its readiness levels, I also encourage Texans in the storm’s path to make their own emergency preparations, heed warnings from local officials, and avoid high water areas,” he said on Wednesday.

Some organizations voluntarily ordered evacuations. At Texas A&M University in Corpus Christi, students were told to evacuate by Thursday and to avoid parts of Southeast Texas, including the Victoria, Houston and Galveston areas.

The police in Aransas Pass, Tex., about 20 miles northeast of Corpus Christi, ordered a mandatory evacuation and said city crews would shut off city water on Friday. Officials in Calhoun County and San Patricio County also issued mandatory evacuation orders.

The Houston Independent School District — the largest school district in Texas — canceled classes on Monday.

And late Thursday afternoon, President Trump weighed in on Twitter, urging people to plan ahead as Hurricane Harvey strengthened.

If the storm were to reach New Orleans, the impact could be particularly troubling. The city is still recovering from flooding caused by a severe thunderstorm this month, after a turbine that powered pumping systems failed. In its latest update on Thursday, the city said it expected up to 10 inches of rain and potential flooding starting late Sunday and potentially lasting through midweek.

When NOAA announced the start of hurricane season in June, there were predictions of as many as five major hurricanes. A hurricane this weekend would be the first test of its kind for the leadership of Brock Long, the new administrator of the Federal Emergency Management Agency, which is responsible for coordinating the government’s relief efforts after natural disasters.

A version of this article appears in print on , on Page A17 of the New York edition with the headline: Under a Hurricane Warning, Coastal Texas Braces for Heavy Flooding. Order Reprints | Today’s Paper | Subscribe